Film casting method

ABSTRACT

A pipe for supplying a dope to a co-casting die consists of a goose neck portion connected to a feed block of the co-casting die, and a straight pipe portion connected to the goose neck portion. The goose neck portion has a radius of curvature of 50 mm to 400 mm, and the straight pipe portion forms an angle of 0 to 20 degrees to the horizontal plane. Venting valves are opened up at the beginning of dope feeding, and are closed to some extent as the dope comes out. After the air is completely vented out, the venting valves are completely closed. Thereby, an air collection is completely let out from the pipe, a plastic web discharged from die lips of the co-casting die would not contain any air bubbles, it is prevented that the web splashes to put on the die lips, causing streaks on the produced film.

This application is a Divisional of application Ser. No. 10/936,570filed on Sep. 9, 2004, now U.S. Pat. No. 7,186,107, and for whichpriority is claimed under 35 U.S.C. § 120; and this application claimspriority of Application No. 2003-320272 filed in Japan on Sep. 11, 2003under 35 U.S.C. § 119; the entire contents of all are herebyincorporated by the reference.

FIELD OF THE INVENTION

The present invention relates to a film casting apparatus for formingplastic film by casting, the plastic film being suitable for opticalapplications. The present invention relates also to a protective film ofa polarizing plate, and the polarizing plate using the protective filmthat is formed by use of the film casing apparatus of the invention.

BACKGROUND ARTS

In recent years, there is an increasing demand for transparent plasticor resin film for optical applications, especially for high-qualityprotection film of polarizing plates for liquid crystal display devices.For the manufacture of such a plastic film, an apparatus using asolution film-forming method is suitable.

The solution film-forming method uses a solution called dope, which ismade by dissolving material flakes with a solvent and adding someadditive agents according to the needs, such as plasticizers,UV-absorbers, antidegradants, slipping agents, and a releaseaccelerator. The solution film-forming method includes the steps ofcasting the dope from a dope supply device, called a casting die, on asupport that is constituted of a horizontal endless metal band or arotary drum, drying the dope on the support to some extent till it getsrigid to be a solidified film, peeling the solidified film off thesupport, and removing the solvent from the film while conveying itthrough a drying section.

On the other hand, in order to develop industrial production of plasticfilms, it is important to improve their productivity. To improveproductivity, it is necessary to automatize as many manufacturingprocesses as possible. Especially for embodying a solution film-formingmethod of manufacturing plastic film, there is a large need forautomatizing a casting apparatus, also because it will preventdischarging organic solvent into the environment and polluting theenvironment. Such an automatic casting apparatus is disclosed forexample in Japanese Laid-open Patent Application No. Hei 8-244053.

However, the solution film-forming method of manufacturing plastic filmor resin film can suffer from a trouble that is caused by the aircontained in the dope. That is, when the dope is initially fed to thecasting die at the start of operation, the air collects in the leadingstream of the dope, and often forms bubbles. If the dope containing thebubbles is cast on the support for the film-forming, the resin solutionor dope will splash and stick to die lips. This will result in formingundesirable streaks on the products.

To solve this problem, the present applicant has invented an apparatusfor feeding the dope free from the air to the casting die, and filed aPatent application for this apparatus, as Japanese Laid-open PatentApplication No. 2000-317961. This apparatus is provided with a secondpipe or a dope tapping pipe in addition to a dope supply pipe for acasting die, in the vicinity of a dope entrance of the casting die. Theflow of the dope may be switched over between the dope supply pipe andthe dope tapping pipe, so that an initial flow of the dope, ascontaining the air, is conducted into the dope tapping pipe. Thereafter,the air-free dope is conduced through the dope supply pipe to thecasting die.

However, it has been found that the apparatus disclosed in JapaneseLaid-open Patent Application No. 2000-317961 cannot completely vent theair that collects in the dope supply pipe, so that it is difficult toautomatize the starting process of the solution film-forming.

SUMMARY OF THE INVENTION

In view of the foregoing, a primary object of the present invention isto provide a film casting apparatus that prevents the discharged dopefrom containing air bubbles, which may be caused by the air collectingin the dope supply pipe.

Another object of the present invention is to provide a protective filmfor a polarizing plate, which is manufactured by the apparatus of theinvention, and a polarizing plate using the protective film.

To achieve the above and other objects, the present invention provides afilm casting apparatus for forming a plastic film by casting a resinsolution from a casting die onto a support, the film casting apparatuscomprises a solution supply pipe for supplying the resin solutioncontinuously to the casting die,

The solution supply pipe comprising a goose neck portion connected tothe casting die and having a radius of curvature of 50 mm to 400 mm, anda straight pipe portion connected to the goose neck portion, thestraight pipe portion forming an angle of 0 to 20 degrees relative to ahorizontal plane.

According to a preferred embodiment, the goose neck portion or thestraight pipe portion is provided with at least an air vent valve.

According to another preferred embodiment, the goose neck portion or thestraight pipe portion is provided with at least an air bubble sensor.

The resin solution is preferably a cellulose ester solution.

According to the present invention, a protective film for a polarizingplate is manufactured by the film casting apparatus of the presentinvention.

A polarizing plate of the present invention uses the protective filmmanufactured by the film casting apparatus of the present invention.

It is preferable that the viscosity of the resin solution is in a rangefrom 30 to 3000 P (Poise), at a measuring temperature of 35° C. and thatthe average speed of the flow of the dope through the solution supplypipe is in a range from 1 to 30 m/min. at the beginning of feeding.

According to the film casting apparatus of the present invention, thecasting di e is connected to the goose neck portion having a radius ofcurvature of 50 mm to 400 mm, and the goose neck portion is connected tothe straight pipe portion that forms an angle of 0 to 20 degrees to thehorizontal plane. This configuration promotes letting air bubbles out ofthe pipe, so that the air will hardly collect in the pipe. Therefore,the present invention prevents mixture of air bubbles into the resinsolution.

As the goose neck portion or the straight pipe portion is provided withthe air vent valve, the air bubbles can be vent out before they enterthe casting die. So the air collection will not be formed inside thecasting die.

Since the resin solution or dope is prevented from containing airbubbles when it is extruded from the die lips of the casting die ontothe support, the dope will not splash to put on the die lips, so theproduct will not get streaks that may otherwise be caused by the dopestuck to the die lips.

Providing the solution supply pipe with an air bubble sensor makes itpossible to monitor the air collection in the pipe, and control the airvent valve depending upon the monitoring results. Then it becomes easyto start the film-forming process automatically. As a result, theproductivity of the plastic film is remarkably improved.

Using the cellulose ester solution as the dope contributes to producinga streak-free high-quality optical application film. The plastic filmcasting apparatus of the present invention can produce a high qualityprotective film for a polarizing plate, and a high quality polarizingplate using the protective film manufactured by the apparatus of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages will be more apparent fromthe following detailed description of the preferred embodiments whenread in connection with the accompanied drawings, wherein like referencenumerals designate like or corresponding parts throughout the severalviews, and wherein:

FIG. 1 is a perspective schematic diagram illustrating a film castingapparatus according to an embodiment of the present invention; and

FIG. 2 is an explanatory diagram illustrating solution supply pipes ofthe film casting apparatus of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As resin raw materials of a plastic web or film by to form by thecasting apparatus of the present invention, following examples can berecited: lower fatty acid esters of cellulose, polyolefins (for example,norbornene polymer), polyamides (for example, aromatic polyamide),polysulfides, polyethers (including polyether sulfide, polyetherketone), polystyrenes, polycarbonates, polyacrylates, polyacrylamides,polymethacrylic acids (for example, polymethyl methacrylate),polymethacrylic amides, polyvinyl alcohols, polyureas, polyesters,polyurethanes, polyimides, polyvinyl acetates, polyvinyl acetals (forexample, polyvinyl formal, polyvinyl butyral) and proteins (for example,gelatin).

Among these materials, lower fatty acid esters of cellulose arepreferable as the film material for optical applications, and cellulosetriacetate is the most preferable.

The resin materials are dissolved into appropriate organic solvent toprepare at least two kinds of resin solutions, hereinafter called thedopes. The organic solvent can be halogenated hydrocarbons (such asdichloromethane), alcohols (such as methanol, ethanol, butanol), esters(such as methyl formate, methyl acetate), and ethers (such as dioxane,dioxolan, diethyl ether).

The cellulose ester solution may be added with many kinds of knownplasticizers, such as triphenyl phosphate, diethyl phthalate, andpolyester polyurethane elastomer. According to the need, well-knownadditives such as UV-absorbers, antidegradants, slipping agent, peelingaccelerator may also be added to the solution.

To prepare the dope, the raw material resins and other components may bemixed and dissolved into the solvent according to a well-known method.Alternatively, a cool dissolving method is usable, wherein the rawmaterial resins and other components are swelled in the solvent, and theswelled mixture is cooled down below −10° C. and is then heated up above0° C. for dissolving.

The viscosity of the dope is normally in a range from 30 to 3000 P(Poise), at a measuring temperature of 35° C., and the average speed ofthe flow of the dope through the dope supply pipe is preferably in arange from 1 to 30 m/min. at the beginning of feeding. The dope is castby use of a film casting apparatus 10 as shown in FIG. 1.

In FIG. 1, the film casting apparatus 10 is constituted of a co-castingdie 12 having a feed block type resin solution gathering device, here inafter called briefly the feed block, 11, and a casting belt 15, whichmoves continuously below die lips 13 of the co-casting die 12, andserves as a support for supporting a multi-layered web 14 as it isdischarged from the die lips 13. The casting belt 15 is an endless beltthat is suspended between a pair of rollers 17 and 18, and is made ofstainless steal. As the roller 17 is driven to rotate, the casting belt15 circulates endlessly between the rollers 17 and 18.

On the casting belt 15, the multi-layered web 14 is gradually solidifiedinto a sheet of film 20 as the solvent gradually vaporizes from the web14. The film 20 is peeled off the casting belt 15 by use of a peelingroller 21, and is fed to a not-shown drying device. After being dried,the film 20 is wound into a roll by use of a not-shown winder. It is tobe noted that the casting belt 15 is just an example, and any otherknown device is usable for drying the multi-layered web 14.

The feed block 11 has a well-known structure, wherein there are acentral passageway and side passageways on opposite sides of the centralpassageway. The central passageway is feeding a dope for forming anintermediate layer. The side passageways are feeding a dope for formingan obverse surface layer and a dope for forming a back layer are fedonto the opposite sides of the flow of the dope from the centralpassageway.

As shown in FIG. 2, the feed block 11 is connected to three pipes 25, 26and 27 to supply the feed block 11 with the three kinds of dopes forforming the obverse, intermediate and reverse layers respectively. Thepipe 26, which is for feeding the dope 28 for the intermediate layer,consists of a goose neck portion 31 coupled to the feed block 11, and astraight pipe portion 32 extending between the goose neck portion 31 anda three-way valve 33.

The goose neck portion 31 has a radius of curvature R in a range from 50mm to 400 mm, and more preferably from 50 mm to 200 mm. The goose neckportion 31 is provided with three air vent valves 35, 36 and 37. Thestraight pipe portion 32 is inclined to a horizontal surface by an angleθ of 0 to 20 degrees. The length of the straight pipe portion 32 isabout 0.5 to 3 m, but it may be defined appropriately. It is found thatair collection 38 in the pipe 26 is prevented by defining the radius ofcurvature R of the goose neck portion 31 and the inclination angle θ ofthe straight portion 32 in the above mentioned ranges.

The three-way valve 33 is connected to, besides the straight pipeportion 32, a pipe 41 for feeding the dope 28 from a not-shown dopesupply source, and a feed back pipe 42 for feeding the dope 28 from thepipe 41 back to the dope supply source, for example, when to interruptthe film casting apparatus 10. The straight pipe portion 32 may partlybe a flexible hose, or may partly include a goose neck portion.

The film casting apparatus 10 configured as above opens the air ventvalves 35 to 37 at the beginning of dope feeding, and closes the airvent valves 35 to 37 to some extent as the dope 28 comes out. After theair completely goes out through the air vent valves 35 to 37, the airvent valves 35 to 37 are completely closed. This way, the air collection38 is completely vent out from the pipe 26 before the dope 28 flows intothe feed block 11. So the multi-layered web 14 as ejected from the dielips 13 would not contain any air bubbles. Therefore, the multi-layeredweb 14 would not splash to put on the die lips 13. So the film 20 wouldnot suffer the streaks that could be caused by the air bubbles in theweb 14.

Disposing a bubble sensor 39, for example, an infrared transmissive orreflective sensor, in the vicinity of the air vent valves 35 to 37 ispreferable for the sake of automatic control of opening and closing theair vent valves 35 to 37. Then it is possible to automatize theactivation of the film-forming process. It is also possible to providean air vent valve in a part of the straight pipe portion 32.

In the above embodiment, the pipe 26 is provided with three air ventvalves. The number of air vent valves is preferably not more than three,because the air vent valve may cause a malfunction when the dope staysin between the air vent valve and the dope supply pipe. For the samereason, it is desirable that the air vent valve has a valve structurewith fewer portions allowing the dope to stay there.

Although the present embodiment has been described with respect to thepipe 26 for the intermediate layer, other pipes 25 and 27 may have thesame structure as the pipe 26, and may be provided with at most threeair vent valves each. So the same applies to any of the three pipes 25to 27.

The casting belt may be replaced with a cooling rotary drum or any otherknown device that serves as a support for the multi-layered web in themanufacturing process for a multi-layered film.

Insofar as it is the feed block type, any kind of dope gathering devicemay be used for continuously gathering the different kinds of dope toform a multi-layered film. The co-casting die may have an appropriateinternal structure. For example, the co-casting die may be a coat hangerdie or a T die.

Although the present invention has been described with respect to theembodiment where the co-casting die is used for forming three layers ofsolution flow, the present invention is applicable to a casting die forcasting a single-layer web.

The film 20 to be produced by the film casting apparatus of the presentinvention preferably has a thickness of 30 to 200 μm, though it variesdepending upon the raw material and the application of the film. Thefilm produced by the film casting apparatus of the present invention isusable as a protective film of a polarizing plate, or a film base forphotographic film. The polarizing plate may be formed by putting theprotective film on either side of a polarizing web that is formed frompolyvinyl alcohol or the like.

The film may also be applicable to other kinds of optical function film,such as an optical compensation film that is formed by bonding anoptical compensation sheet on the film, or as an antireflection filmthat is formed by laminating a glare protection layer on the film. Thesearticles may be used as a part of a liquid crystal display device.

It is preferable to set the flow of the dope through the supply pipe ata greater value than necessary for achieving an expected thickness ofthe film, before the dope is discharged from the casting die. When thedope begins to go out from the casting die, the flow is reduced to avalue that will achieve the expected thickness, and is maintained atthis value while the dope is being discharged. This method is effectiveespecially in the co-casting using a feed block, though it is alsoeffective for casting a single-layer web.

If the flow of the dope is lower than the necessary value for theexpected thickness, the preventive effect against the air bubbleformation will be reduced. If, on the contrary, the flow of the dope isgreater than the necessary value for the expected thickness during thedischarging, it becomes difficult to peel the plastic film completelyoff the support, so the remaining film may spoil the support. Thesupport may be an endless belt or a drum.

If the expected thickness is too small, undesirable slits will be formedin the web at the first stage of dope discharging. If the expectedthickness is too large, the film becomes more likely to remain on thesupport. Therefore, it is desirable to preset the expected thickness ina range from 80 to 400 μm in terms of dried film thickness. In thatcase, the clearance between the die lips is generally 0.5 to 3 mm.

The clearance between the die lip tip and the support is preferably from1 to 20 mm. This clearance is preferably reduced to a necessary distanceafter the start of casting, in order not to stain the lip tips.

The flow rate should be controlled at proper timings while calculatingthe time on the basis of a piping capacity from the three-way valve 33to the feed block 11 and the flow rate. The time of lowering the flowrate is preferably 10 to 2 seconds before the start of discharging, butthis is not limitative.

It is desirable to preset the support at a temperature before the dopedischarging, where the film is easy to peel off. It is also desirable todry the web by blowing it to an extent that would not disturb thedischarge of the dope from the casting die. For example, where the webis cooled for gelatinizing, the support is preferably preset to have asurface temperature of −5 to 15° C., and more preferably 0 to 10° C.Then, the film will be almost completely peeled off the support, whilethe film is kept in tight contact with the support. Therefore, it ispreferable to preset the surface temperature of the support at anappropriate value before the film-forming.

For the casting method where the film is peeled off after the solutionis sufficiently dried, the surface temperature of the support ispreferably set at 10 to 30° C. and more preferably 15 to 25° C. It is tobe noted that the above mentioned temperature ranges of the support aredefined on the assumption that the dope is a solution of triacetylcellulose into methylene chloride. For another kind of solution, thetemperature range of the support should be defined by experiments.

The airflow of the drying air is preferably reduced to 20 to 80%, morepreferably 30 to 70%, of that used generally for manufacturing thearticles. Then, the drying air will not disturb the soft web at the exitof the casting die. It is possible to feed the drying air since severalhours before the start of discharging, or after the dope discharging.The drying temperature is usually set lower than the manufacturingcondition for the articles. For example, if the manufacturing conditionis 40° C. the drying temperature is set at 20 to 30° C. and 40 to 80° C.for 120° C. where as 50 to 90° C. for 140°C. But these values are notlimitative.

Now numerical values and experimental results of examples 1 to 7 of thepresent invention and those of comparatives 1 to 3 will be shown asfollows. But the present invention is not to be limited to theseexamples. These examples and comparatives will be described with respectto the pipe 26 for the intermediate layer, but the same applies to theother pipes 25 and 27.

EXAMPLES

The pipe 26 of the film casting apparatus 10 was formed from atransparent resin, so that the air collection 38 in the pipe 26 may beobserved from outside by naked eyes.

The pipe 26 has an internal diameter of 40 to 150 mm, and morepreferably 50 to 90 mm. In the examples and the comparatives, theinternal diameter of the pipe 26 was set at 55 mm.

The solution density of the dope 28 was set at 1300 kg/m³, and theviscosity of the dope was set at 130 Pascal second. The numerical valuesand the experimental results of the examples and the comparatives areshown in Table 1, wherein “θ” indicates an inclination angle of thestraight pipe portion 32, “R” indicates a radius of curvature of thegoose neck portion 31, “T ” indicates a time lag from the opening of theair vent valves to the start of dope discharging, “L/s ” indicates aflow volume (litter per second) of the dope, and “m/s ” indicates a flowvelocity (meter per second) of the dope.

TABLE 1 θ R T Reynolds (°) (mm) (sec.) L/s m/s number Result Example 1 050 136 3 1.3 0.012 OK Example 2 5 50 136 3 1.3 0.012 OK Example 3 10 50136 3 1.3 0.012 OK Example 4 15 50 136 3 1.3 0.012 OK Example 5 20 50136 3 1.3 0.012 OK Example 6 15 200 144 3 1.3 0.012 OK Example 7 15 400159 3 1.3 0.012 OK Comparative 1 25 50 136 3 1.3 0.012 NG Comparative 225 50 205 2 0.8 0.008 NG Comparative 3 5 50 205 2 0.8 0.008 NG

In any of the examples 1 to 6, the air collection was not observed. Inthe example 7, the air collected a little in the radially outward areaof the vertical portion of the goose neck portion, but it disappearedquickly. In the comparative 1, the air collected in the radially outwardarea of the vertical portion of the goose neck portion, through therewas no air in the outlet of the straight pipe portion. In thecomparative 2, there was the air in the outlet of the straight pipeportion. In the comparative 3, the air collected in the goose neckportion, through there was no air in the outlet of the straight pipeportion. As seen from Table 1, if the dope 28 flows at a speed of 1.3m/second through the pipe 28 where the inclination angle θ of thestraight pipe portion 32 is 5 to 15 degrees, the air collection 38 wouldnot occur in the straight pipe portion 32.

It is also found that the air is more likely to stay in a vertical ordownward portion of the pipe 26, particularly in radially outward areainside the goose neck portion 31, with an increase in the radius ofcurvature R of the goose neck portion 31. It has been found that theradius of curvature R may be set in a range from 50 to 200 mm with noproblem, and the upper limit is 400 mm.

The present invention is not only applicable to a film castingapparatus, a protective film of a polarizing plate, and a polarizingplate, but may also be modified for the use in manufacturing a liquidcrystal display.

1. A film casting method for forming a plastic film comprising the stepsof: supplying a resin solution continuously to a casting die with asolution supply pipe, said solution supply pipe comprising a gooseneckportion connected to said casting die and having a radius of curvatureof 50 mm to 400 mm, and a straight portion connected to said gooseneckportion, said straight pipe portion having an angle of 0 to 20 degreesto a horizontal plane; and casting said resin solution from die lips ofsaid casting die onto a support.
 2. The film casting method as claimedin claim 1, wherein said gooseneck portion or said straight pipe portionis provided with an air vent valve, said supplying step including thesteps of: opening said air vent valve at the beginning of said resinsolution feeding; closing said air vent valve to some extent as saidresin solution comes out from said air vent valve; and closing said airvent valve completely after the air is completely vented out.
 3. Thefilm casting method as claimed in claim 2, wherein said gooseneckportion or said straight pipe portion is provided with an air bubblesensor, and opening and closing of said air vent valve is automaticallycontrolled based on an output of said air bubble sensor.
 4. The filmcasting method as claimed in claim 3, wherein said resin solution is acellulose ester solution.